Liquid crystal displays have adopted a backlighting system which lights the display from the back side thereof. However, in recent years, such a sidelighting system as described in Japanese Patent Laid-Open Publication No. 63-62104 has been widely used due to an advantage that the display can be made thin and can be lit uniformly. This sidelighting system is a system that applies light to the display from cold-cathode tubes or the like from the edge of an acrylic plate or the like having certain thickness, and due to dot printing, illuminating light is dispersed uniformly, resulting in a screen having uniform brightness. This system has an advantage of making the liquid crystal display thinner than the backlighting system because it places light sources not on the back side but on the edge of the screen. However, a reflecting plate needs to be placed on the back side of the screen to prevent illuminating light from escaping to the back side of the screen. Accordingly, the reflecting plate is required to have high light reflectivity and high light diffusibility.
As a method for obtaining a polyester film suited for a liquid crystal display reflecting plate which meets this purpose, a method of incorporating an incompatible resin is described in Japanese Patent Publication No. 8-16175. Although this method is a method which can produce the above film at relatively low cost, it is unsatisfactory in terms of an improvement in reflectivity since it merely adds the incompatible resin, and the brightness of the screen of a produced liquid crystal display is also unsatisfactory. Further, when inert particles such as titanium oxide are added in high concentration, an improvement in reflectivity can be expected. However, when the inert particles are added in an amount of, for example, 50 wt %, the concentration of the inert particles is so high that ruptures often occur and film formation is very difficult. Accordingly, a white polyester film which achieves a good balance between an improvement in reflectivity and ease of film formation has been needed.
Further, for conventional white polyester films, antimony trioxide has been widely used as a catalyst as described in Japanese Patent Laid-Open Publication Nos. 63-137927 and 63-235338. Antimony oxide is liable to be deposited on a die when a molten polyester resin is extruded from the die, and the deposited antimony oxide is liable to produce streak-like defects on the molten resin. In particular, in the case of white polyester films, the streak-like defects are easily observed as black streak-like defects. Therefore, measures therefor have been desired.